The following type of index table is available. In the index table, a setting surface, which is set to a machine tool, is provided at a frame orthogonally to a surface of a rotating table. A motor is disposed at either of two surfaces of respective side surface portions of the frame, the two surfaces being orthogonal to the surface of the rotating table and the setting surface. The motor is connected to a worm shaft orthogonal to this surface through a transmitting element. The motor drives a worm wheel, mounted to a rotating shaft of the rotating table, through the worm shaft, to drive the rotating table.
Specifications of this type of index table are related to the following devices. They are a device based on a right-hand specification, in which the motor is positioned on the right of the rotating shaft when the surface of the index table is seen from the front with the setting surface that is set to the machine tool being set at the lower side, and a device based on a left-hand specification, in which the motor is positioned on the left of the rotating shaft when the surface of the index table is seen from the front with the setting surface that is set to the machine tool being set at the lower side. Either one of these devices is selected in accordance with the machine tool to which it is mounted, to prevent processing of a workpiece from being restricted due to occupied space of the motor.
In this type of index table, depending upon a change in a processing condition such as a change of a machine tool to which the index table is mounted, the type of tool, or the shape of a workpiece, there may be a request at a working place to make it possible to change from the device based on the right-hand specification to that based on the left-hand specification or to change from the device based on the left-hand specification to that based on the right-hand specification. When such changes are made, there is a desire to use the worm wheel and the worm shaft without replacing them.
Therefore, in an index table discussed in Patent Document 1, of four surfaces that are orthogonal to a surface of a rotating table, excluding the surface from which a motor projects, two or three surfaces serve as surfaces that can be set to a machine tool. It is possible to change from the device based on the right-hand specification to the device based on the left-hand specification, and from the device based on the left-hand specification to the device based on the right-hand specification without replacing parts.
However, as mentioned above, when the setting surface that is set to the machine tool is changed from one setting surface to another, the perpendicularity between the surface of the rotating table and the setting surface may change. When the perpendicularity changes, processing precision of the workpiece is reduced. The setting surface that is not used is exposed. This may cause, for example, a tool or a workpiece to impair the state of the setting surface. Therefore, when a setting surface whose surface state is impaired is used, processing precision of the workpiece is reduced. Further, the worm shaft may be provided in the up-down direction, or may be provided above the rotating shaft. This may deteriorate an oil bath extending to a portion where a worm and a worm wheel engage each other, and an oil bath extending to bearings at two ends of the worm shaft. Consequently, wearing of and damage to the oil baths may occur early.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2004-142023